1. Field of the Invention
The present invention relates to the field of liquid crystal display (LCD), and more particularly, to a screening method of a phosphor-based optical film used in a backlight module and a backlight module thereof.
2. Description of the Prior Art
A liquid crystal display (LCD) has been broadly applied for advantages over slimness, power-saving, and low radiation. The LCD mainly comprises an LCD panel and a backlight module. The backlight module is set up in opposite to the LCD panel and provides a display light source to it so that the LCD panel displays images by the light from the backlight module.
Conventionally, there are two types of the backlight module, direct and side-in backlight module. No matter of direct or side-in backlight module, however, a light source as a core decides a display effect of the backlight module, such as brightness, chroma and color saturation of the backlight module. The brightness of the backlight module is adjustable by luminous flux, amount, driving methods of the light source and a framework of optical films in the backlight module. The chroma of the backlight module is suited to a standard by a spectrum of the light source, a transmittance spectrum of optical material and a color filter spectrum. The color saturation of the backlight module, as an additional norm, is mainly used for distinguishing a high-class type (i.e. high-class LCD) and a low-class type (i.e. low-class LCD). A norm of the color saturation of the backlight module in a common low-class type is around 62% to 70%. On the other hand, a norm of the color saturation of the backlight module in a high-class type is over 75%. In addition, uniformity of chromaticity on a backlight module side optimizes visual perception of an LCD. For instance, chromatism of the backlight module inside in a low-class type is limited below 0.010.
As FIG. 1 shows, the side-in backlight module comprises a light guide plate (LGP) 10, a light source 20 close to a light-in side of the LGP 10 and a phosphor powder optical film 30 on a light-out side of the LGP 10. The light source 20 adopts blue light-emitting diodes, and the phosphor powder optical film 30 transforms part of blue light from the light source 20 into red and green light, and then the rest of blue light and the transformed red and green light are mixed to get white light as a backlight source of a backlight module. Compared with the white LED as a light source of the backlight module, the backlight module in FIG. 1 optimizes to fit for a color filter in an LCD to achieve better color saturation and transmittance. A material of the LGP (polymethylmethacrylate, MS, etc) and existence of mesh points in the LGP, however, result in that the LGP observes more blue light so that chroma of the LGP in a direction far away the light source increases gradually, and therefore have a strong impact on uniformity of chromaticity of the backlight module inside.